This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Digital television signal systems represent the new generation of television broadcasting, employing digital signal processing in signal generation, signal transmission, signal reception, and customer display. Globally, several standards for digital television signal transmission have been employed including the Advanced Televisions Systems Committee (ATSC) standard predominantly in the United States and the Digital Video Broadcast—Terrestrial (DVB-T) standard predominantly in Europe and other parts of the world. A variant of DVB-T, known as DVB-Handheld (DVB-H) is a standard for application in small handheld and mobile devices based on the DVB-T standard. Both DVB-T and DVB-H use OFDM as the modulation layer format technology.
OFDM is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency, as compared to conventional frequency division multiplexing (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
An OFDM transmission system, such as is used in DVB-T and DVB-H, splits a high-rate data stream into a number of parallel streams to be transmitted simultaneously over a number of sub-carriers. In the transmitter, the data bits are modulated on the sub-carriers using either a phase shift keying (PSK) or quadrature amplitude modulation (QAM) constellation mapping. The OFDM system may also use a concatenated encoding process employing an outer encoding process residing nearer to the data source in the processing chain and an inner encoding process residing nearer to the signal modulation and transmission.
Advances in error correction systems used as part of the signal coding process have allowed further improvement in signal transmission performance, particularly when coupled with transmission systems such as OFDM. One such error correction system is known as Low-density parity-check (LDPC) coding. LDPC codes are generally recognized as Shannon limit approaching codes, particularly when used with longer code lengths. As a result, LDPC encoding may be used in many signal transmission systems, including concatenated signal coding systems, such as DVB-T and DVB-H.
LDPC coding may not provide optimal coding performance when used with certain signal transmission systems. In particular, LDPC coding may not produce optimal coding performance when used with higher order modulation constellation mapping, such as 16 level quadrature amplitude modulation (16-QAM), 64-QAM, and 256-QAM. The performance degradation may be due to the relationship between the relative reliability of the symbols within the modulation constellation and the error protection provided to the data mapped to each of the symbols. Proposals for improvement exist, including multistage decoding at the receiver and bit interleaved coding of the symbols in the constellation. However, these solutions are limited in capability and, as a result, do not fully address the issues with higher order modulation and LDPC coding. Therefore, it is desirable to create an apparatus and method for LDPC coding and symbol mapping process to improve the capabilities of LDPC coding and higher order modulation formats.